Fracture Behavior Under Mode I Loading in Laminated Composite Materials Repaired with Structural Adhesives

One of the most critical damage modes affecting the structural performance of traditional composite materials, and therefore their durability, is the occurrence of interlaminar cracks (delamination), which are prone to grow under different loading conditions. In this study, the feasibility of repairing carbon fiber reinforced polymer (CFRP) laminates using structural adhesives was experimentally investigated by evaluating the Mode I interlaminar fracture toughness. Two unidirectional AS4 CFRP systems were analyzed, manufactured with epoxy 8552 and epoxy 3501-6 matrix resins. Mode I delamination behavior was characterized using Double Cantilever Beam (DCB) specimens. Three commercial structural adhesives were used in the repair process: two epoxy-based systems, (Loctite® EA 9460™, manufactured by Henkel adhesives (Düsseldorf, Germany), and Araldite® 2015 manufactured by Huntsman Advanced Materials (The Woodlands, TX, USA) and one low-odor acrylic adhesive, 3M Scotch-Weld® DP8810NS manufactured by 3M Company (St. Paul, MN, USA). Adhesive joints were applied to previously fractured specimens, and the results were compared with those obtained from baseline composite specimens. The results indicate that repaired joints based on the 8552 matrix exhibited higher strain energy release rate (GIc) values, approaching those of the original material. The 3501-6 system showed increased fiber bridging, contributing to higher apparent fracture toughness. Among the adhesives evaluated, the acrylic-based adhesive provided the highest delamination resistance for both composite systems.